1. Field of the Invention
The present invention relates to fuel cell separators and to solid polymer fuel cells which include such separators.
2. Prior Art
Fuel cells are devices which, when supplied with a fuel such as hydrogen and with atmospheric oxygen, cause the fuel and oxygen to react electrochemically, producing water and directly generating electricity. Because fuel cells are capable of achieving a high fuel-to-energy conversion efficiency and are environmentally friendly, they are being developed for a variety of applications, including small-scale local power generation, household power generation, simple power supplies for isolated facilities such as campgrounds, mobile power supplies such as for automobiles and small boats, and power supplies for satellites and space development.
Such fuel cells, and particularly solid polymer fuel cells, are built in the form of modules composed of a stack of at least several tens of unit cells. Each unit cell has a pair of plate-like separators with a plurality of ribs on either side thereof that define channels for the flow of gases such as hydrogen and oxygen. Disposed between the pair of separators in the unit cell are a solid polymer electrolyte membrane and gas diffusing electrodes made of carbon paper.
The role of the fuel cell separators is to confer each unit cell with electrical conductivity, to provide flow channels for the supply of fuel and air (oxygen) to the unit cells, and to serve as a separating or boundary membrane between adjacent unit cells. Qualities required of the separators include high electrical conductivity, high gas impermeability, electrochemical stability and hydrophilic properties.
In such separators, the water that forms during power generation often obstructs the flow channels in the separator, lowering the power generating efficiency. A number of efforts have been made to overcome this problem. These include the following prior-art methods:    (1) incorporating a hydrophilic substance into the separator base (JP-A 10-3932),    (2) administering a hydrophilic treatment to the separator surface (JP-A 8-130024, JP-A 8-130025, JP-A 2000-251903, JP-A 2001-93539),    (3) making the overall separator or the flow channel surfaces thereon of expanded graphite (JP-A 2002-110189), and    (4) making the overall separator of a porous material (Austrian Patent No. 389,020).
In above prior-art method (1), efficient migration of the hydrophilic substance to the flow channel surfaces is difficult to achieve. As a result, hydrophilic treatment is not adequately effective and the hydrophilic substance used dissolves out of the separator as an impurity.
Prior-art method (2) above requires a post-treatment step following hydrophilic treatment. This increases the number of production steps and makes the work associated with production more complicated. In addition, the contact resistance of the separator rises due to hydrophilic treatment, lowering the performance of the fuel cell.
In prior-art method (3), the shape of the separator deforms when the units cells are clamped together during assembly of the fuel cell stack. Moreover, because the separator is made at least in part of expanded graphite, it contains acid from the acid treatment carried out during production of the expanded graphite. This acid promotes the elution of impurities and also has an adverse effect on the electrolyte membrane.
In prior-art method (4), when the porous material is porous sintered carbon, the costs associated with machining the grooves rise and the yield of the machining operation falls. On the other hand, when the porous material is porous molded carbon, sufficient strength cannot be achieved.